I don't know why it would jump back to US, but if you're running under UK all the time, you can remove the US choice by going under the apple to System Preferences, then to International, then to Input Menu.

Find US, remove the tick, then go to the bottom and remove the tick next to Show input menu in menu bar.

OR

Click once on the flag in the menu bar and slide down to Open International and remove the ticks.

The top version is longer, but keep it in mind that once the flag is gone, it's the only way to get it back if you want to undo these changes.

With only one flag chosen, however, the menu-bar flag should disappear on its own.